One of the important universal concepts introduced before the Crisis is Krona and the origin of the universe, as explained in Green Lantern Vol 2 40. I don't know if that fits your definition of important.

^Ooh! I thought it was exclusively for Flash 😅

Let's see now. A story that's quite famous if I do say so myself is Superman's Return to Krypton, where Superman meets his birth parents and falls in love with a Kryptonian actress

Another important issue is Superman and Batman meeting for the very first time

I myself just got into the Silver Age not too long ago, so I'm still not so knowledgeable about pre-crisis stories

I was adding all of the golden, silver, bronze age comics and calling them pre crisis sorry for the confusion

I got you. What I refer personally as pre-crisis are the Silver and Bronze Age of DC comics. Golden Age is just Golden age for me

But yeah, I can't really think of anything else. Maybe the first time the Justice League of America teamed up

Also when all Kryptonite was seemingly made inert in Superman: Kryptonite Nevermore

Ok thanks

Entirely depends on what characters or parts of DC you follow. For Superman, some good ones are

• Action Comics #544, which introduced a few noteworthy elements to the Superman mythos such as Luthor's Warsuit and Brainiac's revamp from a goofy, campy character to a much serious and threatening villain. You should read Superman #164 and Action Comics #528 first for greater context however.

• Superman #4, first appearance of Luthor

• Action Comics #242, first appearance of Brainiac and Kandor

• Superman Annual #11, iconic story that's frequently adapted

• Superman #141, was regarded as one of the greatest Superman stories ever written during the silver age

• All-Star Comics #58, debut of Power Girl

• K-Metal from Krypton (lost comic but has a fan reprecation), debut of Kryptonite, and would've drastically changed the Superman mythos far earlier.

• What Ever Happened t the Man of Tomorrow? I wouldn't advise actually read this until you get a decent amount of knowedge of Superman's pre-crisis history, not just the highlights but the little details. This story uses a lot of those bits and pieces to construct its symbolism.

For Batman

• Batman #224, beginning of O'Neal's legendary run and generally considered to be the start of the bronze age. Keep reading from here and you'll eventually see the debuts of Man-Bat and Ra's al Ghul!

• Superman #76, first team-up between Batman and Supes as far as I can tell (first comic on this list I haven't read so idk how good it is)

• Batman #400, last 'proper' appearance of the Pre-Crisis Batman.

Green Lantern stuff

• Green Lantern #16, first appearance of Star Sapphire

• Green Lantern #59, debut of Guy Gardner

• Green Lantern #63, start of O'Neal's GL run, would later introduce John Stewart.

• 1st Issue Special #5, debut of the Manhunters

Misc stuff

• Watchmen ofc

• Secret Society of Super-Villains #1, debut of the Secret Society, also a fun arc where ragtag villains go against Darkseid.

• Last Days of the Justice Society, siimilaar to Whatever Happened to the Man of Tomorrow, serves as a 'conclusion' to the Justice Society.

• Swamp Thing #20 (vol 2), start of Alan Moore's iconic run

• Swamp Thing #37 (vol 2), John Constantine debut

This is FAR from complete but hope this helps.

On the Manhunters note, I'd like to point out that their backstory connected to the Green Lantern mythos was introduced in Justice League of America Vol 1 140 - 141.

Thanks for the clairification!

Also, most of New Teen Titans (1980), featured the most iconic incarnation of the team, which was mostly adapted in the 2003 cartoon (albeit without Kid Flash and Wonder Girl), and would later introduce other well known characters like Blackfire and such.

Most of the Flash's important comics, like most heroes, will be collected in their anniversary trade paperbacks. For example, the Flash has had The Flash: A Celebration of 75 Years and the more recent 80-year anniversary. I'd recommend using them as a starting point, as they introduce major characters and stories.